The invention relates to a method for producing a holding apparatus designed to electrostatically hold a component, in particular a silicon wafer. The invention also relates to a holding apparatus produced by this method. Applications of the invention are in the production of devices or tools for holding components by means of electrostatic forces, in particular for holding semiconductor components such as, for example, silicon wafers.
Holding apparatuses for holding components electrostatically, which are also referred to as electrostatic holding apparatuses, electrostatic clamping apparatuses, electrostatic clamps, ESCs or electrostatic chucks, are known in general. An important application of electrostatic holding apparatuses is in holding silicon wafers in lithographic semiconductor processing, for example in chip production, in which particular advantages of electrostatic holding that are exploited are ease of switching of electrostatic holding forces, high positioning accuracy and reliable fixing of the silicon wafer in a vacuum.
In semiconductor processing, there are particular requirements relating to positioning accuracy. Silicon wafers are typically flexible semiconductor wafers, which have to be held with as perfect a planarity as possible, e.g. with deviations of less than 15 nm from the flat form, for exposures in the lithography process. For this purpose, it is necessary for the holding apparatus to have a flat, free support surface for receiving the silicon wafer, and a high mechanical and thermal stability.
Typically, an electrostatic holding apparatus has a structure having a plurality of plate-type or layer-type elements that perform differing functions. For example, an electrostatic holding apparatus having a plurality of plate-type elements is described in US 2013/0033690 A1. At least one plate-type element (referred to as a core element in the following), for example made of a mechanically stiff ceramic, has a carrying and cooling function. At least one further plate-type element (referred to as a holding element in the following) performs the electrostatic holding function of the holding apparatus. For this purpose, the holding element is equipped with an electrode device, by means of which the electrostatic holding forces are generated. Since the electrostatic holding forces are reduced as the distance from the electrode device increases, the holding element is as thin as possible. In order nevertheless to impart the required mechanical and thermal stability to the holding element, it is fixedly connected to the core element, for example as in US 2013/0033690 A1, by anodic bonding.
The conventional holding apparatus has the disadvantage that the form of the support surface of the holding element follows the surface form of the core element, as a result of being connected to the core element by anodic bonding. However, the core element may have form deviations from the desired form of the support surface, in particular from a flat form, such that the support surface also becomes deformed by the anodic bonding. There may additionally be form deviations, relative to the support surface, on the surface of the holding element that faces towards the core element. Consequently, in the case of the conventional electrostatic holding apparatus, deviations in the form of the support surface, for example from the desired planarity, may occur, that have a disadvantageous effect upon the semiconductor processing of the silicon wafer, in particular in the exposure of the latter. Hitherto, it has only been possible to avoid these problems by elaborate processes for setting the planarity, this having a disadvantageous effect upon the duration of production.
The said disadvantage occurs, not only in the case of electrostatic holding apparatuses for silicon wafers, but generally in the case of all holding apparatuses whose use requires a support surface of highly exact form.